To my untrained eye this looks like M.duwei. M.stella-de-tacubaya is described has solitary, 4-5cm across, 3-4cm wide and the central spine being black. M.duwei is larger all round and does clump in culivation, it also has a red / brown spine. The main different will be in the flower. If you can post a picture once they have opened I am sure that will clear up the matter.